Stimulation fluids are used in the oil and gas industry to create additional permeability in oil and gas reservoirs. The stimulation fluids are typically comprised of a fluid and chemicals that are mixed at the surface and blended to create viscosity, also known as a gel state. The chemicals include a gelling agent, an activator and a breaker. For many hydrocarbon fluids, the gelling agent is a phosphate ester, the activator is an iron base chemical and the breaker is a basic metal oxide.
The viscous fluid is pumped at high rates downhole and is forced into the hydrocarbon-bearing reservoir under high pressures. When the fluid pressure reaches a level that exceeds the compressive rock pressure, the fluid creates fractures in the rock, and expands existing fractures. The pressurized fluid flows into the fractures. As this is happening, sand or other proppants may be then added to the fluid and pumped downhole to stabilize or prop the fracture open and to provide porosity.
As the fracture stimulation ends, the breaker chemical in the stimulation fluid begins to degrade (or break) the gel, which reduces the viscosity to pre-gel levels. Once the hydrocarbon fluid is broken, the fluid (commonly referred to as ‘flow back’) is then brought to surface and subsequently sent to oil recyclers for clean-up and reuse.
The extent of the recycling of hydrocarbon stimulation fluids has been limited. Typically the fluid will be stored and reused by simply increasing the concentration of all the chemicals used to create and break the gel. Reuse in this manner can occur only a limited number of times before the hydrocarbon fluid becomes too unstable to create a usable gel system. Lime has also been used to remove residual gels from the stimulation fluid that have not broken. Heating and filtering processes have also been used to treat the hydrocarbon flow back and to remove any solid particles such as sand and clay.
We have previously described a procedure for treatment of a used hydrocarbon stimulation fluid (in co-pending U.S. application Ser. Nos. 11/422,948 and 11/555,149, the contents of which are incorporated herein by reference) entailing treatment with an acid, allowing the activator and breaker chemicals to react to form an aqueous phase and a hydrocarbon phase. The aqueous phase is removed and the hydrocarbon phase is filtered through a settling agent, such as clay or a fine filter to remove the gelling agent. The steps of treatment to remove the activator, breaker and gelling agents may be carried out in any sequence.
Existing recycling processes typically employ a mixing vat and storage tanks, or a number of tanks connected in series. However, the use of multiple tanks can leads to problems with integrity and maintenance as the number of connections increases with the increased number of the tanks used. Also, clean up of an increased number of tanks results in a longer downtime and reduced productivity. Further, as the prior art operations require sequential agitation, settling and filtering steps, the process time for treatment of the fluid is prolonged.
Therefore, there is a need for an apparatus and method for the treatment of stimulation fluid that mitigates the disadvantages of the prior-art.